1. Field of Invention
The present invention relates to computer printers, and more particularly, to computer printers having devices for clamping the paper as the paper is printed.
2. Description of the Prior Art
In one type of computer printer, the type faces of the characters to be printed are carried on a medium such as a character band or character drum which rapidly moves past a printing station. The printing station typically includes one or more banks of aligned, individually actuable hammers. Disposed between the hammers and the moving character medium is a ribbon and the paper to be printed. Printing is accomplished by actuating each hammer at the appropriate time to propel it against the moving surface of the character medium when the character to be printed moves into alignment with the hammer striking face.
The paper is normally stepped one line at a time to the print station with a full line being printed between successive paper movements. One type of advancement mechanism for stepping the paper is known as a tractor feed which includes a pair of continuous loop belts disposed at the edges of the paper path. Each belt has outwardly extending teeth which engage holes provided at the edges of the paper. As the belts circulate, the paper is drawn forward. One such tractor feed is usually located past or "downstream" of the printing station. Optionally, a second tractor feed may be positioned "upstream" of the printing station and is operated in unison with the downstream tractor feed.
One problem associated with moving character media is that the character medium tends to pull the paper as the hammer strikes the paper against the moving medium, unless the paper is secured as each line is printed. Such movement of the paper can misalign the printing with a corresponding loss in printing quality.
Moreover, the paper often becomes particularly unstable when the end of the paper roll (or continuous folded paper) passes the upstream tractor feed. This instability can significantly disrupt the printing so that the last sheet must be reprinted. Consequently, time is often spent in resetting the starting point of the computer data for the next sheet coming up for printing. In addition, it is possible to lose the misprinted data in some circumstances.
In order to prevent the character band from moving the paper, devices such as spring-loaded clamps have been utilized to clamp the paper as each line is printed. However, because the clamping force typically provided by such clamps often will not allow the paper to be advanced while the clamp is activated, means are usually provided to lift the clamp from the paper after each line has been printed in order to advance the paper another line. After the paper has been advanced to the next line, the clamp is again lowered to the paper to secure the paper for the next printing operation.
Electromagnets have typically been used to lift and lower the spring-loaded clamps for each advance and print step. Such arrangements have tended to be relatively complicated, expensive to manufacture and subject to breakdown.